Welcome to Pigeon Talk.It sounds close to the ruptured airsac idea. Please tell me a little more about that. What is an air sac and how would that rupture. Because I did think it had a lot of excess air in its crop for a few days before this.
I am a novice at pigeons, but not completely in the dark either. It's not the first baby I've nursed.
* I only nurse the ones that are not growing as fast as nest mates, so I know they come with some sort of baggage. But they end up being so friendly it's worth the extra effort.